Humeral Head Resurfacing Implant

ABSTRACT

A humeral head resurfacing implant ( 11 ) that has a modulus of elasticity close to that of human cortical bone as a result of its design from an integral substrate of isotropic graphite covered completely with a reinforcing layer of dense isotropic pyrolytic carbon. A carefully engineered cruciform stem ( 15 ) extends from the axial center of a flat distal circular surface ( 23 ) of a spherical cap portion ( 19 ) of the implant head located within the confines of a surrounding skirt portion ( 21 ).

This application is a continuation of U.S. application Ser. No.13/523,012 filed Jun. 14, 2012, now U.S. Pat. No. 8,690,958, which is acontinuation of international Application No. PCT/US2010/059301, filedDec. 7, 2010, which claims priority from U.S. Provisional ApplicationSer. No. 61/286,284 filed Dec. 14, 2009, the disclosures of all of whichare incorporated herein by reference.

This invention relates to a prosthetic implant designed to function as ahumeral head resurfacing prosthesis. More particularly, it relates to ahumeral head resurfacing implant having an improved design that enhancesstem strength and stem fixation to bone while conserving bone stock andproviding overall bone and cartilage compatibility.

BACKGROUND OF INVENTION Humeral Head Arthroplasty

Disease and injury often require shoulder joint arthroplasty using ahumeral head prosthesis. There are two types of humeral head prosthesesin general use; one is a humeral head resurfacing implant and the othera humeral head replacement implant. Humeral head resurfacing is aconservative approach to humeral head arthroplasty and is usuallyaccomplished by using a thin-wall, dome-shaped shell to resurface thehumeral head. The resurfacing implant has a central stem that is placedin the humeral neck following bone preparation to achieve fixation tothe humeral bone. In humeral head replacement, the entire humeral headis cut off during surgery, and the humeral head replacement implant hasa long medullary stem to fix the implant to the humeral bone.

The shoulder joint is formed by the head of the humerus articulatingwith a shallow socket called the glenoid. The glenoid is located on thelateral margin of the scapula. Humeral head resurfacing and replacementimplants can be total joint implants or hemi-joint implants. Total jointimplants typically have a polyethylene socket component that replacesthe glenoid avid articulates with a humeral head replacement.

More than half of the shoulder joint arthroplasty presently done in theUS are hemi-arthroplasty because exposure to provide access to theglenoid is difficult. The bone mass comprising the glenoid is limited,and because of this, glenoid replacement components often loosen. Forhemi-arthroplasty, humeral head replacement, may be presently morecommonly used than humeral head resurfacing.

A humeral head resurfacing implant was developed by Dr. S. A. Copelandand was first used clinically in about 1986. The Copeland implantconsists of a thin-wall, metal, spherical, dome-shaped shell having acentral tapered and flirted, stem intended to achieve fixation of theimplant to bone.

Humeral head resurfacing arthroplasty has the following benefits whencompared to humeral head replacement.

1. Humeral head resurfacing is a conservative procedure requiring muchless removal of bone than humeral head replacement. In humeral headreplacement, the entire humeral head is removed, and a substantialamount of bone is also removed to make room for the long medullary stemthat extends deep into the proximal shaft of the humerus. In humeralhead resurfacing, only articular cartilage and a small amount of thesubchondral bone is removed from the humeral head to reshape it forreception of the resurfacing implant. In like manner, only a smallamount of bone is removed from the humeral head to make room for theimplant stem. Minimal bone removal is an objective and a benefit ofhumeral head resurfacing. The humeral head is left essentially intactduring the resurfacing procedure; such maintains the integrity andstrength of the humeral head bone structure. Maintaining integrity andstrength of the humeral head bone is important because the nativehumeral head structure provides the foundation needed to support thebiomechanical loads that will be encountered by the humeral headresurfacing implant.

2. The orientation of the humeral head with respect to the long axis ofthe humerus varies considerably from, individual to individual. Withhumeral head replacement, many combinations of head and stem componentshaving different shapes are required to achieve the correct anatomicposition of the humeral head for each individual patient. With humeralhead resurfacing the position and location of the humeral head is notaltered during surgery and the individual anatomy of each patient ispreserved.

3. Substantial intra-medullary reaming is not required. Therefore, thisis a less traumatic procedure in an elderly patient that reduces risk offat embolus or hypotension.

4. If there is malunion (non-healed fracture) at the proximal end of thehumeral with secondary osteoarthritis, the malunion can be leftundisturbed and just the humeral articular surface replaced.

5. With humeral head resurfacing, there is no stem extending down thehumeral shaft, and therefore no possibility of humeral shaft bone lossdue to stress shielding or a stress riser effect that could result, in alow fracture at the tip of the prosthesis.

Humeral Head Resurfacing Surgical Procedure

The surgical procedure consists of making a surgical incision thatprovides access to the glenohumeral joint so that the shoulder can bedislocated and the humeral head exposed. Once exposed, the size andshape of the humeral head can be determined and an appropriate sizeimplant selected. A guide pin is then placed into the humeral head whichserves to orient a cutting instrument that reshapes the humeral head toconform to the concave inner surface of the implant. Following reshapingof the humeral head, a cannulated cutting tool is place over the guidepin and used to form the cavity into which the implant stem will beplaced. The resurfacing implant is then placed onto die previouslyprepared humeral head, and an inspector is then used to firmly seat theimplant in bone resulting in a press fit. A radiograph is then taken toconfirm the implant is properly placed.

Contact of the inner concave surface of the dome-shaped implant headportion with the surgically prepared convex mating bone surface of thehumeral head provides a large load-bearing area to support joint contactloads. Because the contact between the concave inner surface of theimplant and the convex outer surface of the humeral head will not resistrotation of the implant along multiple axes, the implant generallyrelies upon its stem to resist rotation.

Regarding the implant stem, there are two design options. One is amono-body configuration, i.e. a single unit where the stem is anintegral part of the implant; the other is a modular configuration wherestem components of various sizes can be attached to shell components ofvarious sizes. Modular designs are often used for orthopedic jointreplacements as a means to accommodate variations in anatomy from oneindividual to another. However, a modular design necessarily requires asecure means of attaching the stem component to the head component ofthe device, which is most often accomplished using a taper lockingsystem (e.g. cone-in-cone Morse taper). A locking taper inherentlyrequires that additional material to be used to form the implant stem orinner portion of the shell to form the female component of thecone-in-cone connection. The structure necessary for the female portionof the locking taper takes up additional space, and it requires morebone to be removed as compared to a similar mono-body design. Moreover,the need for removal, of the additional bone required for such a taperlock modular design violates the objective of the resurfacing designprinciple, namely minimal bone removal, and as a result, it reduces theload-bearing capacity of the surgically modified humeral head. Thus, fora humeral head resurfacing implant, a one-piece mono-body configurationthat will require substantially less bone removal should be thepreferred design.

Humeral Head Resurfacing Implant Fixation Long Term Fixation

Fixation of the humeral head resurfacing implant to bone can be achievedusing bone cement or by means of material capable of achievingbiological fixation. Bone cement is known to cause chemical and thermalbone damage during insertion resulting bone necrosis and is known tofracture and fragment while in situ. Both of these factors can result inloss of implant fixation. Biological fixation, where living boneattaches permanently to the implant surface, is considered anadvantageous alternative to bone-cement fixation. Biocompatiblematerials, such as titanium, that allow direct bone to implantadaptation resulting in osseous integration, and porous materialcoatings that allow for bone ingrowth and hydroxyapatite (HA) coatingsthat result in a bone to HA bond are means of achieving biologicalattachment. Biological attachment relies on the bone's natural healingability to achieve fixation of the implant.

Primary Fixation

Achieving long term stable biological fixation of implant requires timefor the bone healing process to integrate, grow into or hood bone to theimplant stem. In this regard, achieving biological fixation of implantto bone is similar in principle to the healing of a fractured bone.Following a fracture, a biologic response generates new bone to bridgethe fracture and unite the pieces of the fractured bone. During thefracture healing process, it is necessary that the ends of the fracturedbone are immobile. Immobilizing fracture bones is accomplishedclinically by applying an external cast or using internal fixationdevices such a plates and screws, wires or intramedullary rods. If thefracture fragments are not adequately immobilized during the 6-8 weeksnecessary for fracture healing, it is likely the fracture will not heal,resulting in a non-union (malunion). A requirement for 6-8 weeks ofimmobilization to achieve fracture healing following surgery alsoapplies to achieving biological fixation of an implant. The implant mustbe immobile to allow the bone tissue to integrate, grow into and or bondto the implant stem, if the implant is not immobile during thepost-operative healing period it is likely a secure biologicalattachment of implant to bone will not be achieved.

In the case of a humeral head resurfacing implant it is the stem of theimplant that provides the primary fixation required to achievebiological attachment. The implant stem most be designed to provideadequate post-operative immobilization for a period of 6 8 weeks so thatbiological fixation of the implant stem to bone can be achieved.

Humeral Head Resurfacing Implants in Current Use

Two humeral head resurfacing devices are in common use at this time, oneproduced by Biomet Orthopedics (Copeland implant) and the other producedby DePuy Orthopedics (Global C. A. P, implant).

The Biomet Copeland humeral head resurfacing implant is a mono-bodydevice consisting of a dome-shaped shell having a spherical convex outerarticular surface, a concave inner surface and a central peg shaped stemto achieve fixation in the humeral bone. The device is made of ASTM F-75Co—Cr casting alloy, and the outer convex surface is polished andintended to act as the articulating surface. The inner concave surfaceis intended to bear against the surgically prepared humeral head. Atapered, four fluted stem extends outward from the center on the innerconcave surface of the shell. The stem is inserted into a surgicallycreated cavity made in the humeral head and is intended to stabilize thedevice in the humeral bone. The inner concave surface of the dome-shapedshell has a plasma-sprayed titanium layer to achieve osseous integrationand is available with a plasma-sprayed hydroxyapatite (HA) layer placedon the titanium layer to promote bonding of the implant to bone. TheCopeland Humeral Head Resurfacing Implant is approved for use with andwithout bone-cement.

The DePuy C.A.P. humeral head resurfacing implant is a mono-body deviceconsisting of a dome-shaped shell having a spherical convex outerarticular surface, a concave inner surface and a central peg shaped stemto achieve fixation in the humeral bone. The device is made of ASTM F-75Co—Cr casting alloy, and the outer convex dome surface is polished toact as the articulating surface. The inner concave dome surface has aporous Co—Cr alloy layer intended to bear against the surgicallyprepared humeral head. The tapered stem has a frusto-conical uppersection that extends outward from the center of the inner concavesurface of the dome and a cruciate lower section. The innermost surfaceof the concave dome where the stem connects to the dome is flat; thatis, the concave portion of the dome has the shape of a truncated sphere.A porous coating on the stem extends approximately one half-way down thestem of the implant. The distal portion of the stem has four flutesproviding rotational stability to the implant. The stem is inserted intoa surgically created cavity made in the humeral head and is intended tostabilize the device in the humeral bone. As an added feature to enhancefixation, the C.A.P. implant can be obtained with a hydroxyapatite (HA)coating placed on the porous Co—Cr layer. The DePuy C.A.P. Humeral HeadResurfacing Implant is approved for use with and without bone-cement.

Despite the tact that there are various humeral head resurfacingimplants on the market in the United States at the present time, none ofthem is considered to be totally satisfactory. Other examples of humeralhead resurfacing implants are found in U.S. Pat. Nos. 4,520,964;6,783,549; and 7,517,364 and in Published Application. Nos.2006/0009852, 2007/0156250, 2007/0225822 and 2008/0021564. The humeralhead components of these implants (both for humeral head replacement andresurfacing) in commercial use today in total and hemi-joint replacementare generally made of Co—Cr alloy. It is recognized that Co—Cr alloy isdamaging to joint tissues (cartilage and bone), and this is ashortcoming of such hemi-arthroplasty devices. However, from an overallstandpoint, Co—Cr alloy has become the material of present choice.Pyrolytic carbon (pyrocarbon) has been shown to be much less damaging tonative joint tissues (cartilage and bone); thus, it would be a bettermaterial for hemi-arthroplasty than either metal or ceramics such asaluminum oxide or zirconia. However, pyrolytic carbon has significantlydifferent properties, and as a result has generally achieved commercialuse primarily on articular surfaces.

Accordingly, improvements in such resurfacing implants continue to besought, particularly ones that would utilize pyrocarbon.

SUMMARY OF THE INVENTION

The invention provides a humeral head resurfacing implant formed ofspecific materials and having an improved interior/stem construction,which utilizes a cruciform cross-sectional geometry, that provides bothadequate stem strength and stem surface area without removing excessiveamounts of bone; this implant excellently achieves primary and long termfixation to the resurfaced humerus and provides the benefits ofpyrocarbon. The stem basically employs four fins that extend radiallyoutward in cruciform shape from a center axial region and which areshaped and proportioned to achieve the desired objective of providingadequate strength in a graphite-pyrocarbon structure while requiringonly minimum removal of bone material. The interior construction allowsfin thickness to be minimized while assuring adequate strength inregions of joinder between the stem and the head or cap.

In a particular aspect, the invention provides a humeral headresurfacing implant comprising an integral head and stem which includes:an integral isotropic graphite substrate having a head portion and astem portion of cruciform cross section which extends distallytherefrom, a coating of dense isotropic pyrocarbon having a thickness ofat least about 0.2 mm that covers substantially said entire substrate,which pyrocarbon has a density of between about 1.7 and 2.1 gm/cm³ and ahardness of at least about 200 DPH, said head having an exterior surfaceshape of a section of a spheroid which serves as a proximal surface tointerface with a patient's glenoid or glenoid replacement, and said stemhaving a width of between about 45% and 60% of the width of said head,each of said 4 fins of said cruciform stem, which fins extend radiallyfrom a center axial region, having a thickness equal to between about 2mm and 3 mm and joining said distal surface of said head at a fillethaving a radius of between about 1.5 mm and 2.2 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a cap of a sphere.

FIG. 2 is a perspective view of a humeral head resurfacing implantembodying various features of the present invention.

FIG. 3 is a line drawing similar to FIG. 2.

FIG. 4 is a front view of the implant of FIG. 2

FIG. 5 is a bottom view of the implant of FIG. 2.

FIG. 6 is a sectional view taken along the line 6-6 of FIG. 5.

FIG. 7 is a sectional view taken along the line 7-7 of FIG. 5.

FIG. 8 is an enlarged fragmentary sectional view showing the encircledportion of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Very careful engineering and innovation was required to design apyrolytic carbon-graphite humeral head resurfacing implant that willmeet the strength and performance requirements of such a prosthesiswhile still conserving bone, which is a prime objective of any suchresurfacing implant.

Assuming the articular portion of the humeral head is spherical, itsarticular outer surface can be described as generally that of aspherical cap having the characteristic dimensions of height (H), radiusof curvature (R) and width (W) as illustrated in FIG. 1.

The assumption that the humeral head is spherical is only partiallyaccurate because the native humeral head is actually slightlyoval-shaped, with its base width in the anterior/posterior directionbeing about ninety-two percent of the width in the superior/inferiordirection. However, from an overall consideration standpoint,considering the humeral head to be a segment of a sphere is a reasonableassumption, and the head portion of the humeral head resurfacing implantdescribed herein preferably has an articulation surface that is aspherical cap. The mathematical relationship between H, W and R for acap of a sphere is shown in FIG. 1. Knowledge of any two of thevariables (W, H, or R) allows for calculation of the third.

The biomechanical forces associated with shoulder joint function willdetermine the load-bearing requirement for a humeral head resurfacingimplant. ASTM F 1378 05, Standard Specification for Shoulder Prostheses,states the worse joint load acting on a humeral head can be 2 times bodyweight. Assuming a body weight of 190 lbs., the magnitude of the jointforce acting on the humeral head would be approximately 380 lbs. Thedirection of the joint force will depend on the posture of the arm andthe activity being performed. The force acting on the humeral head willhave components acting in both the axial direction and the lateraldirection. Biomechanical analysis determined that the direction of thejoint force is inclined to the humeral head at an angle of approximately30 degrees for the worse case joint load of two times body weight.

Stem Design Considerations

The implant designer should consider two factors when designing theimplant stem 1) the stem must be strong and durable enough to withstandthe biomechanical forces encountered, and 2) fixation of the stem tobone must be sufficient to withstand the biomechanical forcesencountered. Stem strength is a function of the stem cross sectionalgeometry, and fixation to bone strength is a function of stem surfacearea.

The thickness and radial extension of fins in a stem of cruciformcross-section are most important. A finned stem of careful design canoffer the following advantages:

1. Stem fin configuration provides a large surface area to insureprimary and long term fixation of the implant to bone.

2. Stem fin configuration provides a large radial projected surface areato improve resistance to lateral rotation of the implant.

3. Stem fin configuration provides a large, radial-projected surfacearea to improved resistance to axial rotation of the implant.

4. Stem fin configuration provides for radial extension of the finsallowing the fins to engage denser and stronger peripheral cancellousbone.

5. Stem fin configuration results in enhanced overall stem strength.

6. Stem fin configuration provides high stem surface area to volumeratio that minimizes bone removal and enhances bone fixation.

Cancellous Bone Structure of Humeral Head

The head and neck portion of the humerus consists of an outer layer ofcortical bone and an inner volume of cancellous bone. Cancellous bone isa highly porous or cellular form of bone. The stem of a resurfacingimplant will reside in the cancellous bone structure. Cancellous bone isporous latticework of bony bars and plates where the volume traction ofbone material can range from 5% to 70%. The interstices (void spaces) ofcancellous bone are filled with marrow. The mechanical strength ofcancellous bone is less than dense cortical bone, and the strength ofcancellous bone decreases as its porosity increases. The density ofcancellous bone varies with location. The cancellous bone near thecenterline of the humeral neck is often of low density, and its densityincreases from the centerline to the periphery. This is especially thecase in individuals suffering from arthritis and or osteoporosis wherethe strength of the bone in the center of the humeral head can be verypoor. Because the cancellous bone in the central portion of the humeralhead has lower density and lower strength, it is advantageous to havethe stem fins extend significantly radially out from the bone centerlineto denser and stronger peripheral cancellous bone.

FIG. 2 illustrates a humeral head resurfacing implant 11 embodyingvarious features of the present invention. Its design allows it to bemanufactured from a substrate of isotropic graphite, which haspreviously been used for the production of prosthetic implants ofsubstantial thickness. The substrate is completely covered with a layerof dense, isotropic pyrocarbon of carefully selected physicalproperties, which pyrocarbon layer adds significant structural strengthto the underlying isotropic graphite substrate. The marriage of thesetwo materials and the careful design of the head of the implant and thecruciform stem, and particularly the region where the stem meets thehead, combine to allow the production of an integral implant thatembodies the desired surface properties of pyrocarbon and an overallmodulus of elasticity close to that of human bone.

The humeral head resurfacing implant 11 shown in FIG. 2 is manufacturedfrom a machined substrate of dense isotropic graphite which is thencoated substantially completely overall with a layer of dense, pyrolyticcarbon. This particular combination of materials facilitates theproduction of such a humeral head resurfacing implant having a modulusof elasticity very close to that of human cortical bone. The relativebrittleness of these construction materials is overcome by carefulengineering design that results in a structure which requires onlyminimal removal of bone while still providing an implant with strengthsuch as to allow implantation without damage and assure long-lastingservice and wear for the life of the patient, barring unusualcircumstances.

The machined graphite substrate is formed from dense, isotropic,fine-grain graphite, such as Poco AXF-5Q Biomedical Grade Graphitehaving a density greater than about 1.75 gm/cm³. Preferably, the entireexterior surface of the machined substrate is covered with a layer ofdense pyrolytic carbon, which is chosen for its ability to exhibit highstrength, high wear resistance, biocompatibility, a modulus ofelasticity similar to that of cortical bone, an ability to support boneapposition and low friction on polished surfaces. Pyrolytic carbon whichis isotropic and which has a density between about 1.7 and about 2.1gm/cm³ and a hardness of at least about 200 DPH is employed; such carbonsold under the trademark Pyrolite may be used. However, the preferredcarbon is unalloyed pyrocarbon made in accordance with the teachings ofU.S. Pat. No. 5,677,061, which is commercially available as On-Xpyrocarbon. Such unalloyed pyrocarbon can be obtained having a modulusof rupture of at least about 58 ksi, and pyrocarbon having suchproperties may be preferred as being particularly advantageous for ahumeral head resurfacing implant which will be subject to mechanicalstresses both during implantation and during life in the shoulder regionof the patient.

Such dense pyrocarbon is both stiffer and more fracture-resistant thanthe underlying machined graphite substrate, and as a result of thehigh-temperature pyrolytic coating process, it becomes adhered stronglyto the exterior surface of the isotropic graphite substrate. The resultis one of mechanical reinforcement that provides strength to thecomposite structure and results in an integral implant exhibiting anelasticity modulus very close to that of human cortical bone, which isconsidered to be about 23 gigapascals (GPa). To achieve this mechanicalreinforcement and provide the desired composite modulus of elasticity,the layer of dense isotropic pyrolytic carbon with which the substrateis coated must have a thickness of at least about 0.20 mm (0.008 in.).Preferably, a substantially uniform layer of unalloyed pyrocarbon havinga thickness between about 0.25 mm (0.01 in) and about 0.75 mm (0.03 in)is employed and can provide an elastic modulus within about 25% of thatcortical bone.

The implant 11 includes an integral head 13 and stem 15 that is formedby the integral isotropic graphite substrate that is then coated with alayer of dense pyrocarbon over substantially its entire surface. Thegraphite substrate has a head section in the form of a section of aspheroid and a stem portion which extends from the undersurface ordistal surface of the head portion. The coated substrate thus provides aconvex articular surface 17 in the form of a section of a spheroid,preferably that of a cap of a sphere, which forms the proximal surfaceof the implant that will be polished and will interface with a patient'sglenoid or a with a patient's glenoid replacement should the nativeglenoid require such replacement. Although the head 13 would function solong as it was of a suitable spheroidal shape, for manufacturing andimplantation purposes, a spherical shape is preferably chosen. The head13 includes a cap portion 19, that may be the solid cap of a sphere(FIG. 1) as illustrated in FIG. 6, and an integral, depending peripheralskirt portion 21, which is relatively thin and, in the orientationdepicted in FIGS. 2, 3 and 6, depends below the distal surface of thecap 19 by the amount J (FIG. 6). The skirt 21 has an interior surface 22that is preferably a concave section of a spherical surface that isslightly bulbous at its peripheral edge. The undersurface or distalsurface of the solid cap portion, as best seen in FIGS. 3, 5 and 6, is aflat surface 23, and the stem 15 extends from the precise center of thecircular flat surface 23.

The stem 15 has a cruciform cross section with four fins 25 aligned atright angles to one another and extending radially outward from acentral axial region 27 as seen in FIGS. 3 and 5. The central axialregion 27 has a thickness greater than any one of the four fins 25,which are of equal thickness (T1) (FIG. 4), and it includes four curvedweb regions 29 that are respectively located between each pair ofadjacent fins as best seen in FIGS. 3 and 5. The distal tip or end 31 ofthe stem (FIG. 7) is curved so as to facilitate entry into thecancellous bone region of the humerus head which has been prepared, andthe fins may 25 be tapered longitudinally by few degrees to effect atight press-fit. To secure long-lasting implantation, it is importantthat an excessive amount of bone not be reamed or removed from thehumerus when a humeral head resurfacing implant is used. Accordingly,the implant 11 is designed to have a thin skirt region 21 that limitsthe amount of bone that must be shaved or reamed from the surface of thehumeral head that faces the glenoid. Most importantly, the stem 15 isproportioned and designed to minimize the amount of bone that will needto be removed from the humerus while still providing adequate overallmechanical strength and an overall elastic modulus very close to that ofhuman bone; it provides surface area sufficient to facilitate boneadhesion and secure long-term incorporation into the head of thehumerus. To achieve these ends, it was necessary to very carefullyengineer the implant 11 to arrive at an integral, pyrocarbon-coatedgraphite substrate that would have these attributes and a modulus ofelasticity essentially that of human cortical bone.

It is well known that it will be necessary to produce such implants in avariety of sizes so that a surgeon at the time of implantation canchoose the size that is anatomically the best fit for the repair of theshoulder of a particular patient, and as indicated hereinbefore, it islikely that one would wish to produce these implants having head widthsizes between about 38 mm and about 56 mm. Generally, for implants ofsuch size, the radius of curvature of the spherical head will rangebetween about 18 mm and 29 mm. It is important that the size of the stemis made proportional to the width of the head and particularly tostructurally blend and merge with the interior surface of the skirt 21.The width of the head 13 is labeled (A) in FIG. 4, and the width of thestem 15 is measured lateral edge to lateral edge of 2 coplanar fins andis labeled (C). It has been determined that the stem 15 should have awidth within between about 45% and 60% of the width (A) of the head toassure adequate support for the head of the resurfacing implant andpreferably between about 45% and 50%. If the stem is so proportioned foran implant having a skirt with a thickness T2 (FIG. 6) of about 2-3 mm,the interior region of the head can be sized so that the radial edges ofthe fins of the stem are located so they can carefully transition to theinterior surface of the skirt where a fillet or radius of curvature 33is established therebetween (see FIG. 8) which measures between 1.5 mmand 2.2 mm, and preferably between about 2.0 and 2.2 mm.

Such has proved to be a very important criterion for an implant madefrom these particular materials; however the radius of curvature orfillet 35 at the location where the fins 25 of the stem join the flatundersurface 23 of the cap portion 19 of the head, which is referred toas the distal surface, is likewise important. It has been found that bymaintaining such fillets 33 and 35 within a very narrow range of radiusof curvature, a strong yet acceptable implant is obtained. It ispreferred that radii R1 and R2 be both sized between 2.0 mm and 2.2 mm;more preferably, a radius of 2.1 mm plus or minus 0.03 mm is used toachieve the aforestated objectives. From the standpoint of resistingpotential fracture while minimizing bone removal, the radii of curvatureof the fillets 33 and 35 between the fins 25 and either the flat distalsurface 23 or the interior surface of the skirt portion 21 are ofimportance.

The thickness T2 (FIG. 6) of the skirt, i.e. the distance between thespherical surfaces 17 and 22, may vary slightly as implants range insize from the smallest at a 38 mm head width and the largest at a 56 mmhead width; for example, T2 may be as thin as about 2 mm (0.08 in) forthe smallest size head, but it preferably varies between about 2.4 mm(0.095 in) and 3.0 mm (0.120 in). More preferably, the thickness T2 ismaintained between about 2.5 mm (0.098 in) and about 2.7 mm (0.105 in),and most preferably between about 2.5 mm (0.098 in) and 2.6 mm (0.102in). The skirt may terminate in a slightly bulbous peripheral orterminal region as seen in FIGS. 6 and 8. Careful engineering allowssuch a pyrocarbon/graphite implant to provide necessary strength yetrequire only minimal bone removal that is a hallmark of a humeral headresurfacing implant.

As mentioned before, each of the four fins 25 extends radially from thecentral axial region 27 of the stem, which region is best seen in FIGS.5 and 6. The fins 25 are proportioned to the width of the head 13 sothat the stem 15 will be adequately supporting and afford theaforementioned important transition between the lateral or radial edgesof the fins and the interior surface of the skirt. The fins should havea thickness (T1) between about 2 mm (0.08 in) and about 3 mm (0.12 in)and preferably between about 2 mm and 2.5 mm (0.1 in). It has also beenfound important that the radial width of each fin, i.e. the distancebetween the lateral edge of the fin 25 and the central axial region 21,should be between about 20% to 30% of the head width, and preferablybetween about 20% and 25% of the head width. This arrangement limits thecentral axial region 27 to an acceptable volume while providing adequatestrength.

Overall, the head 13, which includes the skirt region 21, should nothave a height (dimension B) that exceeds about 65% of the height(dimension E) of the overall implant 11. In the FIGS. 4 and 6embodiment, the height of the skirt 21, identified by the reference J inFIG. 6, is equal to about 45 to 65% of the height B of the head 13, andthere is also an important relationship between the height B of the headand the radius R (FIG. 1) of the sphere. The height B of the head shouldalways be less than the radius of the spherical surface 17, i.e. theradius of the spherical cap; thus, the articular surface 19 of theimplant will be less than that of a hemisphere. Preferably, the head 13will be proportioned to have a height B equal to about 55% and about 90%of the radius of the sphere and more preferably between 65% and 90%. Asmentioned before, in the FIGS. 2 and 3 embodiment, the head 13 is acomposite of a spherical cap portion 19 and a depending skirt portion21.

By engineering to the criteria set forth hereinabove, a one-piece,integral humeral head resurfacing implant or prosthesis can be createdby depositing dense pyrocarbon of particular physical parameters onto anisotropic graphite substrate. Although both pyrocarbon and graphite arebrittle materials, careful restriction to these criteria permits thedesign of a implant with sufficient strength and durability to withstandexpected service conditions while taking into consideration certainpertinent factors that need not be considered when one designs implantsof strong Co—Cr alloys or the like. As a result of this carefulengineering and proportioning, one is able to obtain a humeral headresurfacing implant that, while exhibiting most desirable biologicalcompatibility to bone and tissue, i.e. particularly a modulus ofelasticity close to that of human cortical bone, also exhibitssatisfactory biomechanical loading and impact resistance during surgicalinsertion, together with strength and fracture resistance throughout anextended service lifetime.

Although various preferred embodiments are illustrated and describedhereinbefore, it should be understood that changes and modificationsthat would be obvious to one having ordinary skill in this art may beadded without departing from the scope of the invention. Particularfeatures of the invention are emphasized in the claims which follow.

1. A humeral head resurfacing implant comprising an integral head and stem which implant comprises: an integral isotropic graphite substrate having a head portion and a distal stem portion, a coating of isotropic pyrocarbon having a thickness of at least about 0.2 mm that covers substantially said entire substrate, which pyrocarbon has a density of between about 1.7 and 2.1 gm/cm³ and a hardness of at least about 200 DPH, said head of said implant having an exterior surface shape of a section of a spheroid, which surface serves as a proximal surface to interface with a patient's glenoid or glenoid replacement, said head comprising a solid cap section with an undersurface from which said stem extends and a surrounding peripheral skirt section, said stem of said implant being integral with said cap section of said head and having a plurality of flat surfaces, and each of said flat surfaces of said stem being connected to said undersurface of said cap section at a fillet having a radius of between about 1.5 mm and 2.2 mm.
 2. The humeral head resurfacing implant according to claim 1 wherein said skirt section has a height (J) that is not greater than about 65% of a height (B) of said head.
 3. The humeral head resurfacing implant according to claim 2 wherein said height (J) of said skirt section is between about 45% and 65% of the height (B) of said head.
 4. The humeral head resurfacing implant according to claim 3 wherein said head height (B) is about 45% to 65% of the height (E) of the implant.
 5. The humeral head resurfacing implant according to claim 4 wherein said proximal surface of said head is spherical having a radius of between about 18 mm and 29 mm.
 6. The humeral head resurfacing implant according to claim 5 wherein said skirt section has a thickness of between about 2 mm and 3 mm.
 7. The humeral head resurfacing implant according to claim 6 wherein said skirt section has a thickness between about 2.5 mm and about 2.7 mm.
 8. The humeral head resurfacing implant according to claim 3 wherein said stem comprises a plurality of fins having flat lateral surfaces, which fins individually extend from the axial center of said stem, and wherein said fillets where said fins are connected to said undersurface have a radius of about 2.0 mm to 2.2 mm.
 9. The humeral head resurfacing implant according to claim 8 wherein each of said fins has a pair of substantially parallel flat lateral surfaces and a radial width equal to between about 20% and 30% of a width (A) of said head.
 10. The humeral head resurfacing implant according to claim 8 wherein said stem portion of said graphite substrate has fins of an uncoated thickness not less than 1.5 mm.
 11. The humeral head resurfacing implant according to claim 10 wherein each of said fins of said stem has a thickness (T1) between about 2 mm and about 3 mm.
 12. The humeral head resurfacing implant according to claim 11 wherein each of said fins of said stem has a thickness (T1) between about 2.0 mm and about 2.5 mm.
 13. The humeral head resurfacing implant according to claim 7 wherein said skirt section terminates in a bulbous peripheral edge.
 14. The humeral head resurfacing implant according to claim 1 wherein said pyrocarbon coating is unalloyed pyrocarbon having a thickness of between about 0.25 mm and 0.75 mm.
 15. The humeral head resurfacing implant according to claim 1 wherein said undersurface is flat.
 16. The humeral head resurfacing implant according to claim 1 wherein said exterior surface shape of said section of said spheroid is polished. 